Cook buttermilk in a heavy saucepan over medium high heat, stirring often, 4-6 minutes or until bubbles appear around edges (do not boil); remove from heat. (Mixture may curdle).
Lightly beat egg yolks in a large bowl; stir in squash and cheese. Combine cornmeal, and next 4 ingredients in a small bowl. Stir cornmeal mixture into squash mixture. Pour warm buttermilk voer squash mixture; whisk until smooth. Let stand 5 minutes or until lukewarm.
Brush a 2 1/2 to 3 qt baking dish or 12" cast iron skillet with 1 tablespoons mielted butter; stir remaining butter into squash mixture.
Beat egg whites at high speed with an electric mixer until stiff peaks form. Carefully fold into squash mixture. Pour mixture into prepared baking dish.
Bake for 30-35 minutes or until top is golden and a wooden pick inserted in center comes out clean.